It is well known that the photoionization of gases eluting from a chromatograph or leaked into a mass spectrometer provides a useful technique for detecting atomic and molecular species. Although a mass spectrometer is a sensitive analyzer, effective sensitivity is practically limited by a number of processes, including interfering background signals arising from the vacuum system outgassing, the complexities of the fractionation patterns occurring on complex mixtures of high mass number species and the collection of undifferentiated signals due to the detection of metastable species and ultraviolet photons.
The prior art includes a number of techniques designed specifically to reduce background interference. Examples of these techniques are: (1) molecular beam strategies, e.g., U.S. Pat. No. 3,974,380, (2) pulsed input sources, e.g., U.S. Pat. No. 4,365,167 and U.S. Pat. No. 4,855,594, (3) cryogenic techniques, e.g., U.S. Pat. No. 4,039,828, and (4) phase sensitive detection methods, e.g., U.S. Pat. No. 4,258,257 and U.S. Pat. No. 4,263,507. See also U.S. Pat. No. 4,855,594 which utilizes a high density gas sample pulse to sweep residual background from the path of the pulse. However, this method is effective only to about 0.1 ppm.
The ionizers used in these inventions create complex ion fragmentation spectra that often makes it difficult to identify the parent species. This is a significant limitation that compromises the ability of prior-art mass spectrometers to analyze complex organic compounds and biochemical specimens.
The sensitivity of prior art photoionization detectors is further limited in that they cannot detect single ions or very small concentrations of ions. This limitation is due to the fact that these detectors use simple electrodes or Faraday cups to collect the ions and operate at essentially atmospheric pressure; thus, precluding the use of charge-particle multiplier devices which require pressures of less than 10.sup.-4 torr. In general, the local ion density in the detectors belonging to the prior art must exceed 10.sup.5 ions/cm.sup.3 in order to be detected by conventional electrometers in the presence of the shot noise background.
Accordingly, it is an object of the present invention to provide new methods and apparatus for substantially increasing the sensitivity of mass spectrometers and gas chromatographic detectors. It is a further objective of the invention to provide a new photoionization method for vacuum ultraviolet flux irradiation of gas samples.